In recent years, by rapid progress of an MEMS technology (Micro-Electro Mechanical systems), a development of a small mechanical element for electrically displacing or moving a small mechanical element of μm order has intensively been carried out. As the small electromechanical element, there is, for example, a digital micromirror device (DMD) for deflecting light by inclining a micromirror, an optical switch for switching an optical path or the like. DMD has a wide use in a field of an optical information processing such as a projecting display, a video monitor, a graphic monitor, a television set and electrophotography printing and so on. Further, an optical switch is expected to be applied to optical communication, optical interconnection (signal connecting technology by light such as an intercoupling network in parallel computers), an optical information processing (information processing by optical operation) and the like.
With regard to a method of driving a small electromechanical element array of DMD or the like of a background art, there is a description of JP-A-10-48543 shown below. The driving method of the background art will be explained in reference to FIG. 23 through FIG. 25. FIG. 23 is a constitution view of two elements of a small electromechanical element array. A semiconductor board 1 is formed with a driving circuit, not illustrated, at inside thereof, and a surface portion of the semiconductor board 1 is formed with movable mirrors 2, 3.
The respective movable mirrors 2, 3 are supported above hollow spaces by hinges 6 hung between stays 4, 5 erected on the surface of the semiconductor board 1, and made to be pivotable in a left and right direction by constituting centers of pivot by the hinge 6. The hinge 6 is integrally formed with electrode films 7, 8 in the left and right direction interposing the hinge 6, and the surface of the semiconductor board 1 is formed with fixed electrode films 9, 10 at positions opposed to the electrode films 7, 8.
When a bias voltage Vb=24 V is applied as a control voltage to the hinge 6 (electrode films 6, 7) of the movable mirror 2 and an address voltage Va=0 V is applied to the fixed electrode 9 and an address voltage Va2=5 V is applied to the fixed electrode film 10 respectively as element displacing signals, a voltage difference between the electrode films 7, 9 becomes DV=24 V, a voltage difference between the electrode films 8, 10 becomes DV=19 V, by a difference between an electrostatic force between the electrode films 7, 9 and an electrostatic force between the electrode films 8, 10, the movable mirror 3 is inclined in a direction of bringing the electrode films 8, 10 into contact with each other. The illustrated state shows a state of inclining the movable mirror by −10°.
Similarly, when the bias voltage Vb=24 V is applied to the hinge 6 (electrode films 7, 8) of the movable mirror 2, and an address voltage Va1=0 V is applied to the fixed electrode film 9, and an address voltage Va2=5 V is applied to the fixed electrode film 10, a voltage difference between the electrode films 7, 9 becoems DV=19 V, a voltage difference between the electrode films 8, 9 becomes DV=24 V, and by a difference between an electrostatic force between the electrode films 7, 9 and an electrostatic force between electrode films 8, 10, the movable mirror 3 is inclined in a direction of bringing the electrode films 7, 9 into contact with each other. The illustrated state shows a state of inclining the movable mirror 3 by +10°.
When incident light is irradiated to the movable mirrors 2, 3, directions of reflected light differs in accordance with inclinations of the movable mirrors 2, 3, and by controlling the inclinations of the movable mirrors 2, 3, the directions of the reflected light can be controlled to be ON/OFF. However, it is difficult to operate a plurality of mirrors in the same direction or a reverse direction independently and simultaneously and therefore, in the background art, the movable mirror is controlled to be driven by carrying out a complicated voltage control. The control will be explained in reference to FIG. 24, FIG. 25.
A topmost stage of FIG. 24 shows the inclined movable mirror 2. When the movable mirror 2 inclined to a left side is changed to a following state, there are two ways in the “following state”. That is, there are a case of inclining to an opposed side (right side) and a case of inclining to the same side (left side) (case of maintaining an inclined state). To which state the movable mirror 2 is changed depends on image data formed when the small electromechanical element array is used as an image forming apparatus.
Drawings on the left side of FIG. 24 surrounded by a frame at a lower stage show a case of displacing the movable mirror 2 to the opposed side (Crossover transition) and drawings on the right side show a case of maintaining a state of inclining the movable mirror 2 as it is (Stay transition). The address voltages Va1, Va2 applied to the fixed electrode films 9, 10 of the respective movable mirrors 2, 3 are controlled for the respective movable mirrors 2, 3, and the bias voltage Vb is commonly applied to all of the movable mirrors.
When the state of inclining the movable mirror is transited to the following state, the bias voltage Vb is changed as shown by FIG. 25. When a time period from starting to change the movable mirror to finishing to change the movable mirror is divided into zones, Za, Zb, Zc, Zd, Ze, first, at zone Za, the bias voltage is constituted by Vb=24 V, at zone Zb, Vb=−26 V. At next zone in Zc, Vb=7.5 V, at zone Zd, the bias voltage is returned to Vb=24 V, and at zone Ze, the bias voltage is maintained at Vb=24 V.
At zone Za, the address voltages Va1, Va2 are rewritten to 0 V or 5 V. In changing the movable mirror to the following state, when the movable mirror is intended to incline by making the electrode films 7, 8 integrally moved with the movable mirror proximate to the fixed electrode film 9, the voltage Va1 applied to the fixed electrode films 9 is set to 5 V and the voltage Va2 applied to the electore film 10 on the opposed side is set to 0 V. Further, when the movable mirror is intended to incline by making the electrode films 7, 8 proximate to the fixed electrode film 10, the voltage Va2 applied to the fixed electrode film 10 is set to 5 V and the voltage Va1 applied to the electrode film 9 on the opposed side is set to 0 V. Therefore, the address voltages Va1, Va2 are also referred to as element displacing signals.
When the applied voltage is controlled in this way, as shown by the left side (crossover side) of FIG. 24, at zone Zb, the bias voltage becomes Vb=−26 V, the voltage difference becomes DV=33.5 V between the electrode films 8, 10, and the voltage difference becomes DV=26 V between the electrode films 7, 9. Thereby, the movable mirror 2 is applied with an electrostatic force of inclining the movable mirror 2 further to the left side, and the electrode film 8 is further pressed to the fixed electrode film 10 in a state of being brought into contact with the fixed electrode film 10 and elastically deformed. Further, although the state is described as “contact” for convenience of explanation, actually, a gap is maintained between the two electrode films, and the electrode films are not electrically shortcircuited. The same is as follows.
When the bias voltage becomes Vb=7.5 V at next zone Zc, the voltage applied to the fixed electrode film 9 is set to Va1=7.5 V. Thereby, the voltage difference between the electrode films 7, 9 becomes DV=0, and the voltage difference between the electrode films 8, 10 becomes DV=7.5 V. Thereby, the electrostatic force is generated between the electrode films 8, 10, the electrode film 7 is separated from the electrode film 9 by adding a repulsive force by elastically deforming the electrode film 7 at zone Zb to the electrostatic force, and the movable mirror 2 starts rotating in the clockwise direction.
When the bias voltage becomes Vb=24 V at next zone Zd, the voltage difference between the electrode films 7, 9 becomes DV=16.5 V, the voltage difference between the electrode films 8, 10 becomes DV=24 V, the electrostatic force operated between the electrode films 8, 10 is further intensified, and the movable mirror 2 is rotated further in the clockwise direction.
At final zone Ze, the electrode film 7 of the movable mirror 2 collides with the address electrode film 10. At this occasion, the voltage applied to the address electrode film 9 is set to Va1=5 V. The movable mirror 2 is vibrated as shown by a Crossover curve of FIG. 25, and gradually attenuated to be brought into a stable state to thereby finish operation of being inclined to the opposed side.
When the movable mirror 2 is brought into a state of the right side (stay side) of FIG. 24, as shown by an upper stage on the right side in the frame of FIG. 24, the voltage applied to the fixed electrode film 9 is set to Va1=0 V (zone Za). At next zone of Zb, when the bias voltage becomes Vb=−26 V, the voltage applied to the fixed electrode film 10 on the opposed side is set to Va2=7.5 V, and at next zone Zc, the bias voltage becomes Vb=7.5 V.
At this occasion, as shown by a dotted line circle mark CH in FIG. 25, the electrode film 7 is temporarily separated from the electrode film 9, when the bias voltage becomes Vb=24 V at zone Zd, the electrode film 7 is brought into contact with the electrode film 9 again, thereafter, at zone Ze, the voltage applied to the electrode film 10 is set to Va2=5 V, and a state of inclining the movable mirror 2 is maintained in a state of being inclined to the left side. At this occasion, the movable mirror 2 is vibrated as shown by a Stay curve (one-dotted chain line curve) of FIG. 25 by bringing the electrode film 7 and the electrode film 9 into contact with each other, gradually attenuated to be brought into a stable state.
Meanwhile, according to the driving method of JP-A-10-48543, the movable mirror is displaced by operating an external force of the electrostatic force or the like thereto while finely controlling the bias voltage and the address voltages and therefore, the driving circuit and the driving method become complicated. Further, the bias voltage and the address voltages are continued to be applied over an entire time period of a transition time period and a time period of maintaining a displaced state of the movable mirror and therefore, not only power consumption is increased but also a burden on the driving circuit is considerable. Further, vibration is generated at the movable mirror in a final transition time period and therefore, an awaiting time period needs to be provided until the vibration is attenuated and stabilized. In a case in which operation of the movable mirror becomes unstable when the movable mirror is shifted to next operation (displacing operation) during the awaiting time period and therefore, operation at a high speed cycle is restricted.